1. Field
The embodiments discussed herein are directed to a control circuit of a synchronous rectification type power supply unit, a synchronous rectification type power supply unit and a control method thereof.
2. Description of the Related Art
There has been known a synchronous rectification type power supply unit provided with a main switch device and a rectification switch device as disclosed in, for example, Japanese Unexamined Patent Publication No. 2004-135415. In this synchronous rectification type power supply unit, the main switch device connected to a DC power supply is turned ON and OFF cyclically so as to accumulate electric power in an energy accumulating inductance device and a synchronous rectification inductance device connected parallel to the energy accumulating inductance device in an ON period of the main switch device and then the rectification switch device is turned off. Further in this synchronous rectification type power supply unit, when the main switch device is kept OFF, the rectification switch device is turned ON so as to supply electric power accumulated in the energy accumulating inductance device to a load unit.
Before discharge of accumulated electric power of the energy accumulating inductance device is completed, the aforementioned synchronous rectification type power supply unit completes the discharge of the accumulated electric power of the synchronous rectification inductance device, detects the discharge of the accumulated electric power and turns OFF the rectification switch device before the main switch device is turned ON. Consequently, the main switch device and the rectification switch device are never turned ON at the same time, thereby preventing the rectification switch device and the like from being destroyed due to a short-circuit current which flows if both the switches are turned ON.
A synchronous rectification type power supply unit 100 shown in FIG. 5 comprises a main switch device FET1, a rectification switch device FET2 and a comparator COMP 101. In this synchronous rectification type power supply unit 100, the comparator COMP 101 compares drain voltage of the rectification switch device FET2 with grounding voltage. The comparator COMP 101 outputs a high level signal when the grounding voltage is higher than drain voltage of the rectification switch device FET2. The high level signal is inputted to a gate of the rectification switch device FET2 through an output terminal (DL1) of a control circuit 105. Consequently, the rectification switch device FET2 is turned ON.
On the other hand, the comparator COMP 101 outputs a low level signal when the drain voltage of the rectification switch device FET2 is higher than the grounding voltage. The low level signal is inputted to the gate of the rectification switch device FET2 through the aforementioned output terminal (DL1). Consequently, the rectification switch device FET2 is turned OFF.